Electromagnetically operated actuators have at least one electromagnet and an armature which exerts a force on a setting member and which is coupled with at least one resetting element so that the armature may be moved by applying current to the coil of the electromagnet from a first position predetermined by the resetting element, into a second position defined by the abutting relationship of the armature at the electromagnet. Electromagnetically operated actuators are used, for example, for controlling the cylinder valves in piston-type internal-combustion engines. The armature is attached to the engine valve, and the actuator has two electromagnets between which the armature may be moved against the force of a resetting arrangement by switching off the coil current of the holding electromagnet and applying coil current to the capturing electromagnet. By virtue of a suitable actuation of the individual actuators of the cylinder valves an inflow and outflow of the work medium may be achieved so that the work process can be optimally affected dependent on the respective necessary considerations.
The course of the control has a significant effect on the different parameters, for example, the conditions of the work medium in the intake zone, in the work chamber and in the exhaust zone as well as on the events in the work chamber itself. Since piston-type internal-combustion engines operate in a non-stationary manner under widely different operational conditions, a suitable, adaptable control of the cylinder valves is necessary. Electromagnetically operated actuators for cylinder valves are described, for example, in U.S. Pat. No. 4,455,543.
A significant problem in the control of electromagnetically operated actuators of the above type is the timing accuracy which is required particularly for the intake valves in the control of the engine output. An accurate time control is rendered difficult by the manufacturing tolerances, the wear phenomena appearing during operation as well as the various operational conditions, for example, alternating load requirements and alternating operating frequencies, because these external influences may affect time-relevant parameters of the system.
A measure for achieving a high control accuracy consists of applying a relatively high energy for capturing the armature at a magnet pole face. Such a high energy input, however, involves a lowering of the operational reliability because when high energy is used, the problem of armature rebound is encountered in a more pronounced manner. Such a problem is caused by the fact that the armature impacts with a high speed on the pole face and rebounds therefrom immediately or after a short delay. These rebound phenomena appearing in the cylinder valve control adversely affect the operation of the engine.
In the earlier-mentioned known electromagnetic actuator, coil springs having approximately linear spring characteristics are used as resetting springs. The magnets of this arrangement, however, have an exponential force characteristic as a function of the armature displacement which has the result that the magnetic force, in case of a significant distance of the armature from the pole face, may be less than the spring force applied to the armature in that position. Further, as the armature approaches the pole face, both forces are approximately equal and upon further approach of the armature towards the pole face, the magnetic force will become significantly greater than the counteracting spring force. Such an excessive magnetic force towards the end of the armature motion results in an acceleration of the armature and thus an increase of the armature speed which has an adverse effect when the armature impinges on the pole face. In addition to an increased wear and a higher noise generation, there is thus encountered the earlier-mentioned further problem of the armature rebound.